1. Field of the Invention
The invention relates in general to a virtual ground array memory and a programming method thereof, and more particularly to a virtual ground array memory and a programming method thereof capable of solving an array effect.
2. Description of the Related Art
In a conventional virtual ground array memory, adjacent two cells may produce an array effect due to different phases, thereby causing the shift of a threshold voltage and a read window loss or even the read failure.
FIG. 1A (Prior Art) is a schematic illustration showing a conventional virtual ground array memory. Referring to FIG. 1A, a virtual ground array memory 100 includes a first memory cell block 110 and a second memory cell block 120. Illustrations will be made by taking a boundary cell 111 in the first memory cell block 110 and a neighboring cell 121 in the second memory cell block 120 as an example. After the neighboring cell 121 is programmed, a program verifying operation is performed on the neighboring cell 121 in order to verify whether the threshold voltage of the neighboring cell 121 is sufficiently high.
FIG. 1B (Prior Art) shows a threshold voltage distribution of the neighboring cell 121. As shown in FIG. 1B, when the neighboring cell 121 passes the program verifying operation, its threshold voltage is a high threshold voltage 130. At this time, because a leakage current Ileak flowing from the neighboring cell 121 to the boundary cell 111 exists, a sense current Isense received by a sense amplifier (not shown) is unequal to a cell current Icell of the neighboring cell 121.
Consequently, when the boundary cell 111 is being programmed, the leakage current Ileak decreases and the sense current Isense increases because the threshold voltage of the boundary cell 111 is sufficiently high. Thus, the neighboring cell 121 is judged as having the insufficiently high threshold voltage, and the threshold voltage of the neighboring cell 121 is shifted downwards from the high threshold voltage 130 to a high threshold voltage 131. Thus, the read window is reduced or even the read may fail in a more serious condition.